Field of the Invention
The present invention relates to wireless communications, and more particularly, to a method of performing a hybrid automatic repeat request (HARQ) by a user equipment which supports a carrier aggregation, and the user equipment using the method.
Related Art
An error compensation scheme is used to secure wireless communication reliability. Examples of the error compensation scheme include a forward error correction (FEC) scheme and an automatic repeat request (ARQ) scheme. In the FEC scheme, errors in a receiving end are corrected by attaching an extra error correction code to information bits. The FEC scheme has an advantage in that a time delay is small and no information is additionally exchanged between a transmitting end and the receiving end but also has a disadvantage in that system efficiency deteriorates in a good channel environment. The ARQ scheme has an advantage in that transmission reliability can be increased but also has a disadvantage in that a time delay occurs and system efficiency deteriorates in a poor channel environment.
A hybrid automatic repeat request (HARQ) scheme is a combination of the FEC scheme and the ARQ scheme. In the HARQ scheme, it is determined whether an unrecoverable error is included in data received by a physical layer, and retransmission is requested upon detecting the error, thereby improving performance.
In the HARQ, if the error is not detected from the received data, a receiver transmits a positive-acknowledgement (ACK) signal as a reception acknowledgement to report to a transmitter that reception is successful. If the error is detected from the received data, the receiver transmits a negative-acknowledgement (NACK) signal as the reception acknowledgement to report to the transmitter that the error is detected. The transmitter can retransmit the data upon receiving the NACK signal.
In a conventional wireless communication system, only one carrier is considered even if a bandwidth between an uplink and a downlink is configured differently from each other. The carrier is defined with a center frequency and a bandwidth. For example, long term evolution (LTE) based on 3rd generation partnership project (3GPP) technical specification (TS) release 8 is a most dominant next mobile communication standard.
The 3GPP LTE system supports only one bandwidth (i.e., one component carrier) among {1.4, 3, 5, 10, 15, 20}MHz. That is, it may be a single-carrier system. LTE-advanced (A) evolved from LTE may be a multiple carrier system. In order to support a full bandwidth of 40 MHz, the multiple carrier system may use two component carriers having a bandwidth of 20 MHz or may use 3 component carriers respectively having bandwidths of 20 MHz, 15 MHz, and 5 MHz.
On the other hand, a future carrier aggregation system may use a plurality of component carriers having a smaller bandwidth than a full bandwidth. The multiple carrier system may be also called a carrier aggregation system.
Advantageously, the multiple carrier system can support backward compatibility with respect to legacy systems, and can significantly increase a data rate by using multiple carriers.
On the other hand, the number of HARQ processors can be significantly increased in a multiple carrier system in comparison with a single carrier system. That is, the increase in the number of HARQ processors may result in the increase in the number of HARQ processes that can be performed simultaneously. However, a reception buffer size of a user equipment has to be taken into account to perform HARQ. Since the reception buffer size has an effect on manufacturing costs of the user equipment, capability of the user equipment is limited in general.
Due to the limited reception buffer size of the user equipment, even if the number of HARQ processes is increased, a size of a buffer for each HARQ process is decreased, which may result in the decrease in a coding gain.